This invention relates in general to polyimide foams and, more specifically, to closed cell polyimide foams having improved flame resistance.
A number of polyimide compositions having excellent flame resistance and utility as coatings and adhesives are described and claimed in U.S. Pat. Nos. 4,161,477, 4,183,838 and 4,183,139, granted to John Gagliani, one of the inventors of the invention claimed in this application.
The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxoimine. The ratio of oxoimine to dianhydride is preferably in the 2.3:1 to 2.7:1 range and the imidization reaction is preferably conducted at a temperature of 170.degree.-200.degree. C. for 20-60 minutes.
The polyimide forming material is then prepared by dissolving the bisimide in an inert solvent; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, upolymerized mixture of N-substituted cyclic bisimide dicarboxylic acid and diamine which is capable of being converted to a high molecular weight polymer by the application of heat.
The solution is coated onto a surface and polymerized by heating to a temperature in the 177.degree.-316.degree. C. range for 30 minutes to 5 hours. The following is exemplary of the exchange reaction which occurs: ##STR1## where n is a positive integer.
The resulting coating is tough, highly adherent to various surfaces, with very few pinholes or bubbles. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
The prior coating material, however, was not suitable for use in applications requiring a cellular or foam material, since conventional agitation foaming and addition of known blowing agents add to process costs and complexity and are not entirely effective at the relatively high polymerization temperature required.
Co-pending U.S. Patent Application Ser. No. 390,778, filed June 21, 1982, by the inventors of the present application, discloses and claims methods of making modified polyimide/polyimide-amide open cell foams and the product thereof.
In that method, the reaction of an oxoimine with an aromatic dianhydride in a mole ratio of from about 0.05:1 to 1.5:1 produces an N-substituted aliphatic monoimide which is then esterified with a suitable reactive solvent. A suitable diamine is added and the liquid is dried to a film or powder. The dry material spontaneously expands into a consolidated open cell foam when heated to a temperature in the range of about 230.degree. to 320.degree. C. At the lower end of this temperature range, a primarily polyimide-amide foam results, while toward the high end of the range the foam is primarily a polyimide. Flexibility, flame resistance and other physical characteristics vary with the proportions of the two polymers in the final product.
While this open cell foam is excellent for many applications, and can have physical properties tailored for specific purposes, it is not optimum for certain uses. The open cell foam tends to shrink when exposed to flame or high heat. It may absorb water or other liquids and is permeable to liquids and gases. Also, the relatively high foaming temperatures require more complex and expensive heating equipment and the process is wasteful of energy.
Thus, there is a continuing need for a closed cell foam having a combination of flame and heat resistance, resistance to flame-induced shrinking, flexibility and resistance to penetration by liquids and gases. Also, there is a need for reductions in process complexity and energy consumption.